1. Field of the Invention
The present invention relates to the art of fluid conduits, and more particularly to the application of polymer substances to restore, strengthen and substitute for deteriorated and new concrete or metal structures that have to resist corrosion.
2. Description of the Prior Art
Waste water and sewerage are frequently conducted through fluid conduits made of concrete. In some applications, the conduits are made of metal. Sewer systems frequently have reservoirs, containers and ponds, such as settling ponds which also are made of concrete and sometimes of metal. Sewerage and waste water often contain material that is highly corrosive, especially with respect to concrete and metal. The closed system which constitutes most sewerage systems in use presently, creates anaerobic conditions ideal for the proliferation of anaerobic bacteriae which result in hydrogen sulfide. Consequently, waste water pipes, sewage pipes, reservoirs, containers and ponds frequently are the subject of substantial corrosion and deterioration. In addition, corrosive material resulting from the digestive by-products of organisms existing within the waste water and sewer systems cause substantial deterioration of the concrete and metal conduits.
Here in this specification for simplicity of explanation, reference will often be made to "conduits" and "pipes," by which terms I intend to include not only conduits and pipes but also box tunnels and culverts, reservoirs, containers and ponds as discussed above. Such "conduits" and "pipes" as those terms are used herein, include, for example, settling or processing "ponds" at sewerage processing plants, back-up reservoirs and, in general, building structures involved with such processing, unless the text specifies otherwise.
The deterioration frequently takes the form of physical decay of the walls of the conduits so that the thickness of the walls is reduced. Actual amounts of the concrete or metal are literally removed or corroded away over the years. In many instances, sewer and waste water conduits and like structures have walls so thin that the conduits have collapsed under the weight of traffic passing over them, and it has been reported that people have stuck their arms through them with relative ease.
Another undesirable effect of the corrosion and deterioration is that in the weakened condition that thin walls have, the walls crack more easily, resulting in leaks in the sewer system. Such leaks undermine the soil or prepared beds in which the concrete or metal pipes lay. Further, such leaks may contaminate the aquifer, may pollute the water ways and may surface to make the environment above unpleasant, as well.
Some of the more common corrosive materials within sewer conduits are sulfuric acid and hydrogen sulfide, which can turn into sulfuric acid under the conditions frequently found in sewer systems. Their corrosive effects may be readily appreciated. Sewer conduits normally have an anaerobic slime deposit along the wall of the conduit which is submerged below the liquid line or level. Various metal sulfates are among the more common ingredients of this anaerobic deposit. In the substantially liquid waste water, the sulfates reduce to sulfide ions, which combine with the hydrogen in the waste water to outgas above the liquid level as hydrogen sulfide.
Hydrogen sulfide, per se, can be highly corrosive to metals, of which some sewer pipes are made. Many concrete pipes have metal, as, for example, steel reinforcing ribs, beams and like support structure, which are sometimes called "rebar."
Oxygen from the air intakes in the sewer system, such as manholes, as well as oxygen from water condensing on the upper portions of the interior surface of the conduit wall which are not in contact with the liquid being conducted, interacts with bacteria within the conduit above the liquid level. The bacteria produces more hydrogen sulfide. Oxygen from the liquid and condensing from the water in the air reacts with the hydrogen sulfide gas to create the highly corrosive sulfuric acid. The hydrogen of the acid reacts with the calcium hydroxide in the cement of the concrete conduit, and calcium results as gypsum or calcium sulfate, which has been described as a soft corrosion product. The gypsum drops off the interior of the wall itself, making the wall more thin and substantially weakened. In this manner, whole chunks of the concrete wall are removed or vanish, reducing the thickness of the wall.
Restoring such damaged and weakened conduits and pipes in the past has been accomplished primarily through two fundamental methods. In one basic modality, the concrete conduits are entered and fresh concrete is troweled onto the walls to build the thickness back to the original dimension. This method is somewhat problematic in that troweling on the roof of the conduit requires special efforts and time. Frequently, such as in sewer conduits, for example, the repair must be performed within a specified time window. Usually, sewer conduits have a relatively lower flow rate or level during the hours between midnight and the normal waking hours of around seven in the morning. After the normal waking hours, most sewer conduits are so full that entry into the conduit and any repair are not possible.
A second modality involves a process of digging from the top surface down to the weakened conduit. The earth around the weakened conduit is excavated, and a layer of concrete of suitable thickness is poured around the weakened conduit to, in effect, provide a new conduit encasing the old, crumbling sewer pipe. This method has special problems, as well. For example, most such pipes are underneath roadways and streets. Excavating down to the sewer pipe requires removing the surface road or street and diverting traffic often for weeks at a time. The amount of concrete required to encase the old pipe is substantial. The manpower required for excavating, concrete pouring and road and street restoration is substantial, as well. Both in time and material, such repair or restoration is expensive, and the disruption to street traffic is costly to the public.
Other modalities exist, such as for example a method called "slip lining," in which new pipe is inserted within the old, crumbling pipe to strengthen the pipe walls. Such methods reduce the diameter of the pipe or conduit and create problems with connecting lateral pipes joined to the pipe being repaired or restored.
Many of the methods used in the past result in a restored conduit which has a surface facing the effluent made of the same material that deteriorated before. This problem has been addressed by coating or lining the interior facing surfaces after restoration of the conduit, but such liners or coatings have met with only varying levels of satisfaction. Further, many of such methods result in a concrete or metal pipe in which the restoration is rigid or brittle. The brittleness of the restoration itself may soon result in further cracking from thermal cycling and from normal as well as violent ground movement.
It has been known in the past to provide lining for the interior surfaces of such conduits and pipes. See, for example, my U.S. Pat. No. 4,792,493 issued to Vernie L. Belcher and myself. Such linings greatly enhance the ability of the conduit to resist the corrosive effects of the many acids, caustics, toxins and organic material common in waste water and sewer systems. Lining, however, is appropriate only where the fundamental integrity of the conduit wall is substantially unimpaired, and its thickness is sufficiently strong enough to contain the substances being conducted through the pipe or conduit and sufficiently strong enough to support the earth and roadway systems above it. Where the integrity of the conduit wall is diminished, restoration of the concrete or metal structure of the conduit is necessary before a lining is applied. Finally, such restoration in many cases must be accomplished within an existing sewer system essentially while the sewer system is operational. This objective requires that any solution be accomplished during short time windows.
It is desired to provide an improved conduit for fluid conduits and pipes having exposure to highly corrosive substances which is more resilient and which resists corrosion and deterioration in the first instance. It is desired to provide an improved conduit coating for restoring the interior integrity of concrete and metal conduits and pipes having exposure to highly corrosive substances. It is desired, further, to provide such a coating which, when applied, results in an improved inter-material structural bonding with concrete or similarly porous structural material. It is desired, yet further, to provide such a coating which is integral, that is, requires an application of only one layer which, when applied, will have great resistance to such corrosive substances without the application of further lining. It is desired, still further, to provide co-polymer foam pipes which have the same strength as like or larger sized, re-enforced concrete pipe structures. It is a further desire to accomplish these objectives while providing a measure of flexibility to the structure in order to resist cracking and consequent leaks. It is greatly desired and sought to provide such structures which can be applied and cured or set within tight time windows that are available for installation.